The claimed invention relates generally to the field of motor control circuits, and more particularly but not by way of limitation, to an apparatus and method for adaptively controlling a motor by providing multiple parallel controller paths (gears) which are adaptively selected to accommodate different load conditions of the motor.
A disc drive is a data storage device for storing digital data. The disc drive magnetically records digital data on circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are rotated by a spindle motor at a constant high speed. The data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably positioned by an actuator assembly.
Disc drive spindle motors typically have a multi-phase direct current (dc) brushless motor configuration. The phase windings are arranged about a stationary stator on a number of radially distributed poles. A rotatable spindle motor hub is provided with a number of circumferentially extending permanent magnets in close proximity to the poles. Application of current to the windings induces electromagnetic fields that interact with the magnetic fields of the magnets to apply torque to the spindle motor hub and to cause rotation of the discs.
A particular disc drive product can have several different configurations to satisfy different capacity/price point needs. For example, a fully populated drive in a particular series might have two discs and four heads, whereas a depopulated model might have the same overall mechanical structure but only have a single disc and two heads. As would be expected, increasing the number of discs increases the required current to rotate the spindle motor at a given operational speed.
The use of motors with hydrodynamic bearings (fluid-bearing motors) adds another option to the disc drive configuration. While providing smoother operation and lower noise, a spindle motor with hydrodynamic bearings typically requires more current to drive the motor than an equivalent spindle motor that uses ball bearings. Moreover, spindle motors that use hydrodynamic bearings experience a temperature effect where the current required to drive the spindle motor increases substantially at lower temperatures due to the viscosity characteristics of the lubricant within the bearings.
Disc drive manufacturers typically qualify multiple sources (vendors) for system components to assure adequate availability and cost levels. Slight variations in operational performance can occur in spindle motors from different sources as well as in spindle motors from the same source but from different manufacturing lots.
These and other factors have affected the ability of disc drive designers to optimize the control electronics (spindle control loop) used to control rotation of the spindle motor. An optimal setting for one configuration of spindle motor may not provide enough current or enough control resolution for another configuration. One approach to overcome this problem involves releasing different firmware code sets to cover each possible combination. While ensuring that the code for each different configuration will be optimized, logistical considerations make implementation of this approach difficult and costly during manufacturing.
Another approach is to implement a single set of firmware code that covers all of the different possible configurations and environmental conditions (such as temperature) and rely upon detection techniques such as taught by U.S. Pat. No 6,057,981 issued to Fish et al. or U.S. Pat. No. 6,078,158 issued to Heeren et al. to select the correct spindle control loop parameters during operation. While viable, limitations with this approach include the required complexity of the code to account for numerous different mechanical and operational configurations of the drive, as well as the possibility for error in the detection operation, leading to a mismatch of spindle loop parameters.
In accordance with preferred embodiments, a disc drive is provided with a spindle motor configured to rotate a magnetic recording disc at a constant high speed.
Motor control circuitry is provided having parallel first and second controller paths. Each path includes a controller and a digital to analog converter (DAC). Each controller preferably has a proportional and integral (PI) controller configuration and establishes an integrator value in relation to a motor speed error signal and an integrator value. The controllers use the integrator values to generate first and second reference signals.
The DACs output respective first and second motor current adjustment signals over respective, different first and second dynamic ranges (gains) in response to the reference signals from the respective controllers. Exemplary values include a fullscale interval of 0.1 volts for the first dynamic range and a fullscale interval of 0.2 volts for the second dynamic range. Each range provides different maximum current values and different resolutions (volts per count).
Any number of a plurality of controller paths can be used as desired. In accordance with preferred embodiments, the circuit operates to select the first controller path to modulate an amount of drive current applied to the motor. Upon sensing the first controller path to be operating within a selected tolerance of a minimum or maximum value of the respective dynamic range, the circuit switches gears so that the second controller path is used to modulate the current provided to the motor. The circuit configures the second controller path in relation to the final existing state of the first controller path to reduce introduction of transients in the motor. Such configuration preferably includes calculation of a new integrator value for the second PI controller. When the PI controllers output pulse width modulation (PWM) signals, such configuration also preferably includes selection of an initial PWM duty cycle for the second PI controller in relation to the final PWM duty cycle of the first PI controller.
The circuit operates to repetitively verify whether the selected controller path is adequate for the existing load of the motor, and changes to a new controller path as needed to maintain desired stability and resolution in the system. The use of multiple controller paths (gears) accommodates a large variation in spindle motor current and a large number of different spindle motor configurations. The circuit also effectively compensates for changes in operational conditions (such as temperature) without the need for external sensing since gear changes are based on the operational conditions of the currently selected controller path.
These and various other features as well as advantages which characterize embodiments of the claimed invention will become apparent upon reading the following detailed description and upon reviewing the associated drawings.